Our Friends

Jan 20, 2009

Way back in the 1910s and 20s, before the invention of radar, Britain needed a way to try and detect incoming unfriendly aircrafts. And solution to this was to build huge, concrete, acoustic mirrors ,whose purpose was literally to "hear further". Believe it or not, the "listening ears" actually worked, and could effectively be used to detect slow moving aircrafts before they came into sight.
Inside the structure, a trained listener would detect the distance and direction of the incoming aircraft as far as 20 miles away. They worked by concentrating sound waves towards a central point, where a microphone would have been located. But the development of faster aircrafts made them less useful, as an incoming aircraft would be within sight by the time it had been located. Apart from this, the operators also found it difficult to distinguish between incoming aircrafts and passing boats and increasing ambient noise made the mirrors harder to use successfully. In any case, they quickly became obsolete due to the invention of radar in 1932. Several were built along the south and east coasts, but few of these remain today. There are three acoustic mirrors in the Denge complex, each consisting of a single concrete hemispherical reflector.

The 200 foot mirror:

The 200-foot mirror is a near vertical, curved wall, 200 feet long. It is one of the only two similar acoustic mirrors in the world, the other being in Maghtab, Malta.

The 30 foot mirror:

The 30-foot mirror is a circular dish, similar to a deeply curved satellite dish, 30 ft across.

The 20 foot mirror:

The 20 foot mirror is similar to the 30 foot mirror, with a smaller, shallower dish 20 ft across. The design is close to that of an acoustic mirror in Kilnsea, East Riding of Yorkshire.

For the restoration in 2003, English Heritage secured £500,000 from the Aggregates Levy Sustainability Fund and from the EU's Interreg programme under the Historic Fortifications Network, as administered by Kent County Council